HARROW: Researchers at ETH Zurich have now developed a new type of acoustic imaging device which, rather than producing a photorealistic image of an entire object, shows only its contours and edges. “This type of measuring method delivers similar results to the edge detection filter in an image-processing software, which allows the outline of prominent photo objects to be identified with the click of the mouse,” explains Chiara Daraio, Professor of Mechanics and Materials. Her method, however, is not software-based. Instead, it extracts the contour information during the acoustic measurement stage.
To understand just how this acoustic edge detector works, it is important to know that sound waves are reflected off edges in a remarkable way: The acoustics near the edges is dominated by so-called evanescent waves. These waves have a much shorter wavelength than the incident sound waves that producte them. As the evanescent waves decay very fast as they propagate they can only be measured in close proximity to the edge. Methods to recover information contained in these waves have been developed in the past; however, the ETH researchers have now devised a new method that intensifies the evanescent waves and differentiates them from larger sound waves that are reflected in the “normal” way.
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